The present invention relates to an insulated gate semiconductor device (insulated gate transistor) such as a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor) or the like, and particularly to a power insulated gate semiconductor device capable of improving reliability of an insulated gate semiconductor device whose protective functions are provided on a chip.
An insulated gate field effect transistor referred to as a "power MOSFET" and capable of handling large power, which has an overheating cutoff circuit internally provided on the same chip to improve the reliability of the insulated gate field effect transistor, has been disclosed in Japanese Patent Laid-Open Publication No. 63 (1988) -229758 as an illustrative example. In the conventional example, a resistance and a zener diode are series-connected between an external gate terminal and an external source terminal. A constant voltage is generated across the zener diode. A resistance is electrically connected to one of both ends of the zener diode, which is located on the external source terminal side, and a diode is electrically connected to the other thereof which is located on the external gate terminal side. In this condition, the temperature is detected by detecting a variation in a fixed or variable fraction of the applied voltage, which is obtained by the resistance and the diode. When devices or elements are overheated , a MOSFET for a protective circuit, whose gate and source are respectively connected to both ends of the resistance, is turned on to cut off or interrupt the power MOSFET that is a main body. In the conventional overheating cutoff circuit, a variation in the gate -to-source voltage of the n-channel MOSFET for the protection circuit becomes large with respect to a variation in the external gate voltage. Therefore, the variation in the gate voltage is liable to relate to a variation in the overheating cutoff temperature.
Japanese Patent Laid-Open Publication No. 63(1988)-229758, Japanese Patent Laid-Open Publication No. 5(1993)-21787, Japanese Patent Laid-Open Publication No. 62(1987)-143450, Japanese Patent Laid-Open Publication No. 5(1993)-304450, Japanese Patent Laid-Open Publication No. 63(1988)-229757, U.S. Pat. Nos. 5,025,298 5,128,823 disclose insulated gate transistors.